Removal of carbon from substrate surface

ABSTRACT

The invention is a method of removing materials such as carbon and metallic elements from a substrate surface via heating in an atmosphere of molecular chlorine and steam. In a preferred embodiment, carbon residue is removed from the surface of a Si or GaAs substrate material.

This is a Continuation of prior U.S. application Ser. No. 08/625,673filed on Mar. 29, 1996 now U.S. Pat. No. 5,998,305.

FIELD OF THE INVENTION

The present invention relates to removing carbon from substratesurfaces. More particularly, the invention relates to a method by whichthis may be accomplished, whereby a steam-chlorine gas thermal cleaningprocess is used.

BACKGROUND

The current method of semiconductor device fabrication involves the useof a light sensitive polymer known as photoresist. A layer of thephotoresist material is spun onto a semiconductor substrate surface at athickness of about 1 micron (10,000Å) and a patterned exposure performedthrough a previously prepared photomask. The wafer is then developed asin normal photography. After developing, the unexposed areas ofphotoresist are washed from the surface leaving selected areas of thewafer exposed or covered, depending on the intent of the fabricator. Thewafer is now ready for a process step, such as the bombardment of thesurface with boron ions to introduce a doping atom into the siliconwafer to change electrical properties. After this process step iscompleted, the developed areas of photoresist must be removed to allowthe sequence to be repeated with a different pattern.

The current semiconductor industry standard stripping and cleaningmethod involves the use of at least three liquid chemical solutionsknown as “SPM”, “SC-1” and “SC-2”, in use since 1970. “SPM” stands for“sulfuric acid peroxide mix,” and “SC” stands for “standard clean”. SPMis a solution of concentrated sulfuric acid and 30% hydrogen peroxide,and is used to remove heavy organics, such as photoresist. SC-1 is asolution of 29 wt/wt % ammonium hydroxide, 30% hydrogen peroxide anddeionized water. It is used at approximately 70° C. to 80° C. to oxidizesurface organic films and remove some metal ions. SC-2 is a final rinsesolution of 37 wt/wt % hydrochloric acid and 30% hydrogen peroxide anddeionized water. It is used at approximately 75° C. to 80° C. Thesesolutions were first developed at the RCA corporation during the 1960'sand are sometimes known as “RCA cleans”. This approach may beaccomplished at temperatures less than 100° C., which is an importantconsideration as uncontrolled dopant diffusion in the wafer itself willoccur if temperatures of approximately 150° C. to 200° C. are reachedfor any extended period of time.

The liquid process is deficient in that safety, environmentalconsiderations of disposal and water availability are major drawbacks.In addition, a more important limitation comes from the inherent surfacetension of these materials. The liquids have difficulty in enteringfeatures smaller than approximately 0.3 micrometers. Finally, the liquidprocess is relatively time consuming because there is a drying steprequired to remove the liquid cleaning agents. This results in lowthroughput. As device and feature size continue to decrease in size, newmethods of stripping and cleaning must be found.

The most straightforward approach to dealing with the problemsassociated with liquid cleaning is to develop gas phase methods. Gasesare easier to dispose of by scrubbing, have less volume, do not requirea drying step and do not have the same surface tension drawbacks. Thisapproach is known as “dry cleaning”.

Initially, gas phase methods similar to those used to removecontaminants such as dust were applied. These techniques use appliedthermal or UV energy for contaminant excitation in an air, oxygen orinert gas atmosphere. (See U.S. Pat. Nos. 5,024,968; 5,099,557 toEngelsberg). Unfortunately, these systems lack sufficient energy toremove photoresist or very heavy contamination.

This problem was partially solved by the use of various excimer laserphotoresist stripping processes such as those disclosed in U.S. Pat. No.5,114,834 to Nachshon, and in WO9507152 to Elliott et al. Nachshonteaches the application of a laser at an angle perpendicular to thesemiconductor surface to remove photoresist via ablation. A reactivegas, such as oxygen or ozone may be provided to react with the ablatedmaterial. Elliott teaches the application of a laser at an angle whichis preferably 15° to the semiconductor surface to remove feature edgesof photoresist via ablation. It is recommended that two applicationsshould be used, wherein the disc is rotated 90° between the first andsecond. As with Nachshon, a reactive gas such as oxygen or ozone may beprovided to react with the ablated material.

These processes are deficient however, in that they leave a residue ofcarbon on the wafer surface which is on the order of 100Å to 200Å indepth (for a 1; micron photoresist layer). Srinivasan et al (J. Appl.Phys. 61(1) January, 1987) teach that the source of the carbon does notappear to be from redeposition of partially combusted ablated carbon,but from a type of ashing process produced from the instantaneous high(>1000 K) temperatures and pressures (>100 atm) of the laser itself.This residue must be reduced to a thickness of less than about 4 Åbefore the next fabrication step can be performed. Attempts to removethe carbon residue with a UV light and ozone treatment have not beensuccessful in part because the UV laser light is absorbed by the ozone.In addition, metallic contaminant residues, such as Al or Fe may alsoresult from the semiconductor fabrication processes. Activated chlorinegas has been used to remove these residues, but this results in damageto the wafer.

OBJECTS OF THE INVENTION

It is therefore an object of the invention to develop a process thatremoves undesired carbon materials from a substrate surface withoutaltering said substrate the surface of the semiconductor substrate. Thisprocess should be low temperature (less than or equal to 200° C.) andalso be strictly gas phase.

It is a further object of the invention to provide a process wherebysuch carbon residues may be removed to an extent such that the nextfabrication step in semiconductor device manufacture is possible.

SUMMARY OF THE INVENTION

Our invention is a method which removes carbon materials from asubstrate surface. This method comprises heating said substrate in anatmosphere of steam and molecular chlorine gas.

In preferred embodiments, the heating is at a temperature of less than200° C., and the steam and molecular chlorine gas are in a ratio ofapproximately 12:1 in said atmosphere.

In another preferred embodiment, the carbon removal treatment iscombined with a photoresist laser ablation step in order to provide amethod for the removal of photoresist from substrate materials.

BRIEF DESCRIPTION OF THE DRAWING

Other objects, features and advantages will occur to those skilled inthe art from the following description of preferred embodiments and theaccompanying drawing, in which:

FIG. 1 shows an apparatus which is used to remove carbon residue whichremains after the application of the laser in the stripping process.

DETAILED DESCRIPTION OF THE INVENTION

The invention is a treatment of a Si or GaAs wafer which has beenstripped of photoresist via laser ablation. As stated above, presentlaser stripping processes are ineffective in that they leave a carbonlayer having a thickness of more than about 100 Å on the surface of thesubstrate. The thickness of this layer must be reduced to an extent suchthat further device fabrication is possible. Thus any carbon remainingmust be at a level such that it cannot be detected by SIMS analysis(e.g. the carbon must be at or below the “background” level ofapproximately 4 Å). The process we have developed accomplishes thisresult, and involves heating the wafer in an atmosphere of steam andmolecular chlorine gas.

Our process uses the following chemical reaction:

2C1 ₂+2H₂O+C→4 HCl+C0 ₂.

This gas phase reaction is remarkable and critical to the processbecause of its ability to remove a layer of carbon at a rate of about1-2 Å per minute at temperatures below 200° C. If the reaction is run athigher temperatures, the substrate materials are etched by the chlorine.

The reaction is carried out on wafers that have been stripped of theirphotoresist by laser ablation. The process may be used to remove acarbon residue of any thickness up to about a micron, the onlylimitation being the time. The process may be carried out in one of twoways.

In the first method, the reaction is carried out in the laser strippingapparatus. In this case, a mixture of humidified argon is blended withmolecular chlorine and delivered to the gas reaction box. Manydifficulties were encountered which involved corrosion within in theapparatus, as the reaction chamber cannot be heated and gaseous HCl wasformed from a photochemically induced side reaction between the steamand chlorine.

In the second method, the reaction is carried out in a speciallymodified thermal furnace, the details of which are shown in FIG. 1. Asshown in the figure, quartz tube 1 is placed in Lindberg furnace 2 whichis ultimately connected to a water source 3 and a chlorine gas source 4.The furnace is shielded to external light sources as stray UV light mayresult in the formation of gaseous HCl. The general process will now bedescribed.

A controlled amount of deionized water flows from the reservoir 3through valve 5 to three-way needle valve 6. At this point it is mixedwith an inert carrier gas (nitrogen) provided from source 7, and flowcontrolled via valve 8. The water is vaporized in stainless steel coil 9and carried via the carrier gas via three-way valve 10 for blending withan amount of chlorine gas from source 4 which has been measured viavalve 11. The blending occurs in line 12 which should be Teflon® so asto prevent corrosion. The use of the three-way valve 11 is beneficialfor three reasons. First, it allows for the confirmation of steamgeneration; second, it controls the application of steam to the wafertreating process; and third, it allows for uninterrupted steamgeneration when changing sample wafers.

Prior to mixing with the steam, the chlorine gas is heated to preventcondensation. This is controlled via temperature monitor 13 and VARIACcurrent controller 14.

The combined water-vapor/molecular chlorine gas stream 15 is introducedinto the quartz tube 1 to treat the wafer 16. Post treatment condensate17 is drained away from the wafer to avoid contamination. The primaryadvantage of this apparatus is that it is far cheaper and easier tomaintain than the laser stripping unit described in the first method.

The steam-chlorine reaction should be run at a temperature which isgreater than 100° C. and less than 200° C. At temperatures of 100° C.and below the steam will condense to a liquid phase which will damagethe wafer. As noted earlier, at temperatures greater than 200° C., theremay be diffusion of ion dopants (such as B or As) into the substrate.

A molar ratio of 12:1of steam to molecular chlorine at a flow rate of 1standard liter per minute is most preferred as this was found to giveoptimal results. Other ratios between 1:1 and 15:1 may be used, but allresult in lower reaction rates which simultaneously extend process timeand potentially increases the amount of substrate etching that occurs.As such these are less desirable. The 12:1ratio is optimal as we foundthat a large excess of water was required for the reaction to proceed ata reasonable rate such that 1-2 Å/min. of carbon are removed. Further,at ratios in excess of 12:1, the chlorine was diluted to such an extentthat processing time was unacceptable.

Also, while the most preferred flow rate is about one standard liter perminute, slower flow rates may be used. These are less desirable becausethey increase removal time. Faster flow rates of up to 20 standardliters per minute do increase the removal rate to some degree, but aremore difficult to sustain.

Our most preferred conditions are:

Temperature: 110-200° C., preferably 150° C.

Blend: 1:1to 15:1, preferably 12:1 (molar) steam: chlorine

Flow: 100-30,000 cc/min, preferably 1200 cc/min steam

50-1500 cc/min, preferably 100 cc/min chlorine

50-1000 cc/min, preferably 50 cc/min

carrier (N₂)

Rate of removal of C: about 1-2 Å/min.

The carrier gas is used to facilitate the movement of steam through thefurnace, and may be any gas which is inert to the steam/chlorinereaction, such as N₂, Ar or air.

It should be noted that other possible applications of this reactioninclude the intentional etching or surface preparation of wafers thathave graphite SiC or diamond layers as well as substrates consistingentirely of those materials. In these applications the temperature ofthe reaction is not limited to the optimum range discussed above. Thereaction is used to remove carbon from the substrate materials so as toetch the substrate. In addition, the process may be used in the removalfrom a substrate of trace contaminant elements such as Al or Fe,resulting from semiconductor device processing.

Specific features of the invention are shown in one or more of thedrawings for convenience only, as each feature may be combined withother features in accordance with the invention. Alternative embodimentswill be recognized by those skilled in the art and are intended to beincluded within the scope of the claims.

What is claimed is:
 1. A method of etching the surface of a SiC,graphite or diamond substrate, said method comprising heating saidsurface in the absence of UV light in an atmosphere of steam andmolecular chlorine gas to remove carbon.
 2. A method of removingphotoresist from the surface of a substrate, said method comprising thesteps of: a) applying a laser beam to said surface such that photoresistis ablated from said surface and a carbon residue remains; then b)removing sold carbon from said surface via heating said wafer in theabsence of UV light in an atmosphere of steam and molecular chlorinegas.
 3. The method of claim 2, wherein said substrate is heated to atemperature of less than 200° C.
 4. The method of claim 2, wherein saidsubstrate is heated to a temperature of more than 100° C.
 5. The methodof claim 2, wherein said substrate is heated to a temperature of 150° C.6. The method of claim 2, wherein said steam and molecular chlorine gasare in a molar ratio of approximately 1:1 to 15:1 in said atmosphere. 7.The method of claim 2, wherein said steam and molecular chlorine gas arein a ratio of approximately 12:1 in said atmosphere.
 8. The method ofclaim 2, wherein said heating takes place in a furnace.
 9. The method ofclaim 2, wherein the substrate is selected from the group consisting ofSi and GaAs.